Efficient power allocation schemes for OFDM-based cognitive radio systems

A power allocation problem to maximize the capacity of the secondary user subject to an interference constraint in OFDM-based cognitive radio networks is studied in this paper. One main problem is how to select subcarriers for allocating power. In this paper, a water-level approach is proposed to obtain the optimal solution by using the linear search method. The proposed optimal scheme is free of the log operation and easily implemented to achieve optimum performance. We also propose two suboptimal schemes with a lower complexity which can select subcarriers of relatively better channel states and causing the less amount of mutual interference. A novel ratio parameter is introduced to indicate the quality of a subcarrier. The superiority of the proposed schemes is verified by simulations. Their performances are better than those of the existing heuristic schemes and very close to that of the optimal solution.